3 minute read

Magnet Forensics is running a weekly forensic CTF. More information can be found on their blog. It is a fun way to practice, so let’s get to it!

CTF Posts

Week 1 Week 2 Week 3

Getting Started

Download the Android data from here. Note that the data was released October 5th, 2020 @ 11AM ET.

Calculate the hash of the suspect data:

sha1sum MUS_Android.tar 
10cc6d43edae77e7a85b77b46a294fc8a05e731d  MUS_Android.tar

Week 4 question:

Chester likes to be organized with his busy schedule. Global Unique Identifiers change often, just like his schedule but sometimes Chester enjoys phishing. What was the original GUID for his phishing expedition?

Oh, that Chester! So again, we have a bit of a riddle. We are looking for 1) scheduling information that 2) has Global Unique Identifiers (GUID), that 3) change based on some action.

Hypothesis 1

H1: Whatever calendar app is installed. I am guessing H1 because it’s a likely place for some schedule info.

I know that some calendar information for Android can be found in com.android.providers.calendar -> databases -> calendar.db. However, I don’t remember seeing the event data. Let’s check it out!

The calendar.db “Calendars” table shows Chester’s main account and some basic calendar info. At least we know he has a calendar registered, and it looks like it is set to sync (sync_events = 1).

The “Events” table is empty. Awww. Notably, there is an _id column, but I’m not sure if that would be a GUID anyway.

There is also com.google.android.calendar. In the files directory, we can see a sync log (sync_history). In the databases directory we can see cal_v2a and cal_v2a-wal. Surprisingly, I couldn’t find much about these files online.

Looking into the cal_v2a database, the Calendars table shows Chester’s account. The Events table, again, is empty—no indication of a GUID for EventId.

cal_v2a-wal seems to contain a lot of information about events for settings from the calendar.

7A  102065892684377886557king.chester.802@gmail.com

Some of these, like 1579627255066000, are timestamps. I would bet this is the default setting information for the calendar. I didn’t see anything that looked like an event and no GUID.

Taking another quick look around, I don’t see any other calendar-specific apps. So I’m going to mark H1 not supported.

Hypothesis 2,3

H2 an alternative app has scheduling capabilities that use GUIDs for events.

While investigating H1, I came across a file 000003.log. Specifically, I did an “Exact Match” keyword search for “Calendar.” This log file contains a list of calendar names, dates, and timestamps. As well as GUIDs!!

That log is located in com.evernote - OH OH! Taking a look in the databases folder, we find user213777210-1585004951163-Evernote.db. This DB contains the table “notes”, where we find Phishy Phish phish with guid “c80ab339-7bec-4b33-8537-4f5a5bd3dd25”.

It looks like H2 is confirmed, and the app is “evernote”. H3 is that this note GUID is the new id, and we need to find the old id.

Using the first part of the GUID (c80ab339) as a substring keyword search will probably return only a few false positives, if any. Indeed, we get 6 hits. One of them is log_main.txt which sounds pretty interesting! log_main.txt is found in com.evernote -> files -> logs.

Our first hit with c80ab339 shows an entry for GUIDSWAP:

2020-03-23 20:08:58.178 D/SyncService: {SyncServiceWorker-2} - GUIDSWAP: Uploaded new note (7605cc68-8ef3-4274-b6c2-4a9d26acabf1 -> 

If I didn’t know any better, I’d say that was the modification of a GUID. It looks like GUID 7605cc68… was changed to c80ab339… and uploaded.

Trying 7605cc68-8ef3-4274-b6c2-4a9d26acabf1 as an answer… BING!

Lessons learned

  • Even if you start in the wrong place, it gives you intelligence for the next stage.
  • When in doubt, keywords.